Cellular phones, media players, and other high-performance portable electronics are becoming increasingly ubiquitous for business and personal use. Battery technologies have afforded major conveniences for a wide array of portable electronic products. Additionally, battery-powered power tools and home appliances are becoming more commonplace. These systems typically rely on high power density, secondary (rechargeable) batteries such as lithium-ion, nickel-metal-hydride, or nickel-cadmium. In contrast, low-power electronic devices, such as remote controls, medical devices, pagers, watches, and toys, typically use primary (single-use) alkaline manganese, mercuric oxide, silver oxide, or lithium batteries because of their lower energy demands. For modern portable electronic devices, the use of secondary (rechargeable) batteries charged by grid-supplied battery rechargers is energy-inefficient, while the use of primary (single-use) batteries yields disproportionate amounts of hazardous waste.
With improvements in wireless communications and emerging technologies such as wearable electronics, the expansion of portable electronics in modern society shows no sign of slowing. Thus, a careful examination of the energy and environmental impact of these devices is warranted. Poor energy efficiency of batteries, especially for rechargeables, can have detrimental environmental impacts and product safety issues. For example, while the power consumption of an individual device may be relatively small, the total power required to repeatedly recharge hundreds of millions of these devices is not. Currently in the U.S., for example, the total number of cellular phones in use today requires a continuous average power of nearly 300 megawatts, the equivalent of a mid-size power plant. Only 5% of this energy is actually used to power the phone, while the remaining 95% is wasted by inefficiencies in the charging of rechargeable batteries.
In addition, the batteries required to power these portable devices often contain potentially hazardous chemicals, such as mercury, lead, manganese, lithium, and cadmium. These chemicals can contaminate the environment when not properly disposed. Moreover, many of the batteries are prone to overheating, fire, and even explosion. Despite increased efforts for recycling programs, most batteries end up in landfills. It is estimated that only 3-5% of dry-cell batteries are recycled each year [Rechargeable Battery Recycling Corporation Rechargeable Battery Recycling Corporation Website: http://www.rbrc.org/call2recycle/].
The total grid-supplied energy consumption and environmental impact of portable electronic devices can be significantly reduced by developing energy harvesting technologies as an alternative, “green”, power solution. Moreover, completely “unplugging” portable/wireless devices from the hassle of wall-outlet based battery recharging offers the opportunity for enhanced functionality. There exists a number of potential energy harvesting technologies, such as solar, thermoelectric, and vibrational technologies. For personal electronics carried in a pocket, briefcase, or purse where light or temperature gradients are not readily available, vibrational energy harvesters have been of great interest to enable self-powered sensors, wearable computers, and other battery-less electronics [e.g. T. Starner “Human powered wearable computing,” IBM Syst. J., vol. 35, nos. 3/4, pp. 618-629, 1996; R. B. Koeneman, I. J. Busch-Vishniac, and K. L. Wood, “Feasibility of micro power supplies for MEMS,” J. Microelectomech. Syst., vol. 6, no. 4, pp. 355-362, December 1997; S. Roundy, P. K. Wright, and J. Rabaey, “A study of low level vibrations as a power source for wireless sensor nodes,” Computer Communications, vol. 26, pp. 1131-1144, 2003], especially for applications where physical motion is readily available.
Unfortunately, vibrational energy harvesting devices have not found widespread success, largely because of limited performance and poorly targeted applications. In particular, the utility of many previously developed energy harvesters has been limited because the devices have been designed without regard to a particular application, often leading to good perfoimance on a laboratory bench, but no “real-world” application. Regarding performance of the vibrational energy devices, many of the energy harvesters have been constructed using micromachining or micro-electromechanical systems (MEMS) fabrication technologies (intended for ultra-miniaturized self-powered sensor applications). However, with reduced size scale, it has been quite challenging to achieve a mechanical resonance low enough to match the frequency range for naturally occurring vibrations (˜1 Hz-1 kHz). In addition, many previously reported systems are fairly high-Q resonant devices designed for maximum performance at only one narrowly defined frequency. This can limit their usefulness because naturally occurring vibrations usually have wide spectral content, and the primary vibration frequencies change in time. There has been some interest in investigating multi-source (e.g., vibrational, theiinal, and/or solar) energy harvesters [e.g., N. Sato, H. Ishii, K. Kuwabara, T. Sakata, J. Terada, H. Morimura, K. Kudou, T. Kamei, M. Yano, and K. Machida, “Characteristics of thermoelectric and vibrational devices for integrated MEMS power generator,” in Proc. 5th Int. Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Apps. (PowerMEMS 2005), Tokyo, Japan, November 2005, pp. 5-8.].
Accordingly, there is still a need for a multi-transduction method and device for harvesting vibrational energy in an efficient manner with high power density. Such a method and device can reduce the energy demand and environmental impact of portable electronic devices, while simultaneously enhancing the utility for the end user.